Lava caves where young Klingons undergo a ritual of endurance, and where the boy Worf had a vision of Kahless foretelling an unprecedented destiny.
The Caves of No'Mat are a site of Klingon ritual mentioned in the Next Generation episodes Birthright, Part I and Rightful Heir.
When Data asks about visions, Worf describes a rite he underwent as a youth. He spent days in the lava caves fasting in the heat, and Kahless appeared to him and said he would do something no Klingon had done before.
The memory returns in Rightful Heir. On Boreth a man claiming to be Kahless knows of the vision, which helps to persuade Worf for a time that he is genuine. The clerics later admit they created him as a clone and gave him what they knew of the old stories.